The invention relates to a method of filling a weighing container for plastics granulate, wherein the granulate is dropped from a supply tube into the weighing container via a shutter.
A method of this type has been disclosed in DE 10 2004 016 756 B4.
Weighing containers are used in the plastics processing industry for precisely dosing the amounts of material employed in a process of forming plastic melts from granulates or granulate mixtures. To that end, the weighing container is filled with granulate up to a maximum fill level. When the shutter has been closed, the granulate is allowed to exit at the bottom end of the weighing container into a conveyer or a downstream equipment such as a plasticiser, while measuring the decrease in weight of the weighing container and its contents.
Below the shutter there has so far been provided a vertical inlet pipe, via which the granulate enters into the weighing container but which is not rigidly connected to the weighing container and therefore does not contribute to the measured weight of the latter. The weighing container is filled only up to a level at which the repose cone formed by the granulate is still below the lower end of the inlet pipe. In particular, this prevents that the granulate dams up in the inlet pipe and is supported laterally at the vertical walls of the inlet pipe and thereby compromises the result of the weight measurement.
Thus, even at maximum fill level, there is a certain dead space in the top end of the weighing container, and the dead space is not filled with granulate. Granulate dust and so-called “angel hair” may accumulate in this dead space. The term “angel hair” designates filament-like agglomerates of granulate dust which form due to collisions between dust particles when the granulate is conveyed in a vacuum conveying system. Due to their static charge, granulate dust and angel hair tend to adhere to the walls of the weighing container.
In those regions where the dust and the angel hair are not stripped-off by mechanical contact with the granulate particles, i.e. in particular in the dead space at the top end of the weighing container, dust and angel hair may accumulate until larger amounts of this material break away and drop into the granulate. Since this dust-like material is sometimes not melted completely in the extruder, it may result in defects in the plastic products formed from the granulate, in particular in the production of plastic films. It is therefore necessary to remove the dust adhering to the walls of the weighing container in regular intervals. In particular, the dust must be removed when there is a change in the type of the granulate material, in order to assure a uniform composition of the material.
It is time-consuming and cumbersome to remove the granulate dust manually.